ahaha, I would do it too if I have the money~ But..... It's just gonna cost WAY too much. There's trade off going to AWD though. You will have better "racing" launch but bad everyday stop to stop launch since Legends are too heavy and you'll be rubbing off the already "lack of low end". Second, you'll loose the most important "high speed cruising" ability of the Legend because AWD would mean a lot of power loss in the drivetrain when you are at high speed. Better route for an AWD is to get an Audi if you want the Luxury, Subaru if you want the pure mechanical AWD, and Evo VII next year if you want a AWD system that's somewhere in the middle of mechanical and electronical. Notice that those nice AWD vehicles all have max torque in the low-mid engine rev range.
if you want all wheel drive, get subaru wrx or impreza...their AWD is tight...like it hugs the road...me and my friend would always go out to the mountain winding roads and he'd be racing through them no problem and would fishtail on purpose sometimes just to show how easy it was to regain control...but of course these aren't made for top speed...but they are sweet.
Cheapest way to possibly do this would be to use electric motors on each rear wheel. If you have seen your car on a lift you would realize there is no way to get any engine power routed back there. If you integrated the electrical engines to the normal engine management...ala Honda DualNote youd have something. As DV8 mentioned.....10-20K, you are better of with something made for this purpose.
there is also something that has been done to Cadillac Coups for years. Front and Rear Engine. Basically you just put another engine and trans in the back. it is hard to work out, but verry tight. If you got the $$$ and are interested, go for it.